Thermally conductive coatings for light emitting devices

Details Thermally conductive coatings for light emitting devices Thermally conductive coatings for light emitting devices

1998-03-13

2002-05-28

Sanghavi, Hemang (Department: 2874)

Coherent light generators

Particular active media

Semiconductor

C372S045013

Reexamination Certificate

active

06396864

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention is directed to light emitting devices, and more particularly, to coatings for the facets of such devices that provide for high thermal conductivity, in part through lower thermal resistance, to enhance the transfer of heat away from the high temperature, beam emission area of the facet to improve device lifetime and reliability.
2. Related Art
Over the past fifteen years or more, much has been discussed about the passivation, hermeticity or protection of facet surfaces of laser diodes, particularly relative to the front or emitting facet. The emitting facet is also commonly referred to as the output facet of the laser diode. Passivation is the process of protecting the facet from environmental or ambient effects, particularly to oxidation, by isolating the facet from the environment. A coating is applied to the facet and its thickness is adjusted to obtain a desired level of light reflectivity at the light emitting device facet. The need to adjust the coating thickness to achieve the desired level of facet reflectivity is well known in the art. A coating may be applied to the facet surface having a thickness of &lgr;/4 where &lgr; is the laser wavelength of operation, so as to yield a low reflectivity and to enhance the lifetime of the laser. Films, such as SiO
2
or Al
2
O
3
, are typically used as such protective coatings and are deposited directly on the facet surface. Also, the published art discusses problems relating to chemical stability, such as the effect of facet erosion due to the high intensity output of optical power at the facet as well as facet passivation treatment. An example of such passivation treatment is disclosed in the 1982 U.S. Pat. No. 4,337,443 of Umeda et al. entitled, SEMICONDUCTOR LASER DEVICE WITH FACET PASSIVATION FILM. In patent '443, it is recognized that conventional passivation may not provide satisfactory protection against facet erosion attributable to photo-chemical processes that erode the facet, the result of which decreases the laser reliability. The increase in facet erosion is suppressed by the employment of an insulating film of an amorphous material that contains silicon and hydrogen as indispensable elements (&agr;-Si:H). The thickness of the coating material is in the vicinity of &lgr;/4n, where &lgr; is the laser wavelength in the material and n is the refractive index of the film, providing for maximum power output. In another patent in 1989, U.S. Pat. No. 4,815,089 to Miyauchi et al., discloses the use of a single layer of Al
2
O
3
or SiO
2
on the output facet for passivation, having a thickness preferably of &lgr;/3, to properly select a reflectivity in the range of 10% to 20% and provide a stabilized out-put at high powers. The concept disclosed in Patent '089 is to select the proper thickness of the dielectric film to suppress problems relating to increased threshold, astigmatism and optical feedback noise.
In another 1989 patent to Miyauchi et al., U.S. Pat. No.4,860,305, an external cavity laser is disclosed where the rear facet is optically coupled with an external cavity for eliminating longitudinal mode hopping. The emitting facet includes a single film of Al
2
O
3
, which is likely provided for the reasons given in Patent '089, of achieving stabilized single longitudinal mode control at higher powers in spite of aging effects.
In yet another Miyauchi et al. patent, U.S. Pat. No. 4,951,291, the emitting facet is provided with a multi-layer, dielectric coating to provide a protective coating so that oxidation of the front facet can be suppressed to attain an increase of the life span of the laser. The coating comprises a first layer of Al
2
O
3
and a second layer of &agr;-Si:H
2
which is effective for providing high reflectivity, such as 30% or less, as well as suppressing oxidation.
In the foregoing disclosures, only issues of passivation and chemical stability are addressed. Interestingly, no mention or discussion as to the effects of optical power at the facet output and its contribution to the development of high temperatures at the area of beam emission at the facet, the thermal conductivity of the facet coating and its relation to coating thickness to achieve lower thermal resistance, the causes of different photo-chemical reaction rates in facet degradation, how catastrophic optical damage (COD) occurs and can be suppressed to increase device lifetime, how thermal conductivity might be taken into account in the development of facet coatings relative to desired materials for coatings as well as coating thicknesses, and the consideration of thermal conductivity in combination with passivation and chemical stability. U.S. Pat. No. 5,422,901 to Lebby et al. employs a high thermal conductive layer in the form of diamond-like carbon (DLC) surrounding a vertical cavity laser (VCSEL) device, but does not deal with the horizontal cavity laser with an end cleaved facet having a high density output beam, i.e., there is no discussion is made of the development and employment of high thermally conductive coatings at the beam emission area to lower the device temperature at this area to enhance device lifetime and reliability. Moreover, heating is due to high current operation of the vertical cavity device and not due to optical heating of the facet due to optical absorption of a high intensity beam formed by a diffraction limited aperture provided by horizontal cavity, cleaved facet, edge emitting laser device. Also, there is no disclosure or suggestion of how to accomplish efficient heat removal from the output facet of a light emitting device with a cavity emission from a cleaved facet.
A limiting aspect of high-power single-mode and broad area light emitting devices, such as semiconductor lasers, is catastrophic optical damage (COD). COD is a thermal runaway event occurring at the emitting facet of a light emitting device. COD is a function of operational temperature of the light emitting device, the cavity width and length of the light emitting device as well as the current density and optical power density at the output facet. Facet aging leads to increased optical absorption due to surface oxidation or other chemical reactions and, ultimately, to COD which limits the lifetime or reliability of the device. Various methods for postponing the aging process by passivating the output facet have been proposed for prolonging the onset of COD as suggested by the previously discussed disclosures. High quality passivation, however, is often difficult to achieve. Moreover, absorbed optical power at the device facet is what causes a temperature rise at the facet, leading to high facet temperatures that rapidly increase in reaction rates for facet degradation mechanisms, such as chemical or photo-chemical erosion and passivation coating degradation or breakdown over time, or decrease of COD level, all due to such high facet temperatures, shortening the life time of the light emitting device. The invention herein represents an approach for achieving lower facet temperatures for increasing the COD power before, during and after facet aging, extending the life of the device through proper coating of the device facets while concurrently maintaining proper reflectivity as well as providing facet passivation and chemical stability.
Thus, it is an object of this invention to provide a coating for facets of a light emitting device that provides for a lower facet temperatures during device operation by more effectively carrying away heat developed at the facet suppressing the material onset of temperature dependent facet degrading mechanisms occurring at the facet surfaces so that higher power outputs may be achieved with improved device reliability and lifetime.
SUMMARY OF THE INVENTION
According to this invention, a light emitting device is provided with a coating that will increase the thermal conductivity at one or more facets of the device to provide for lowering the facet temperature during device operation to suppress the occurrence of temperature d

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